In the past there have been conventional image-forming devices, such as printers, facsimiles, and copy machines, having sheet postprocessing devices which first stacked sheets such as transfer sheets with images formed thereon, and then performed postprocessing such as stapling, hole punching, and stamping.
Examples of this kind of image-forming device with sheet postprocessing device are disclosed in U.S. Pat. No. 5,385,340 (hereinafter "Document I") and in Japanese Unexamined Patent Publication Nos. 59-82263/1984 (hereinafter "Document II") and 5-286281/1993 (hereinafter "Document III").
For example, the image-forming device disclosed in Document I, as shown in FIG. 6, is composed of a sheet postprocessing unit 304 attached to the side of a main body 301 of the image-forming device.
In this image-forming device, an original placed on an original feed unit 303 is transported to and read by an optical unit 317, and a toner image is formed in an image formation unit 316 according to the Carlson process. A transfer sheet (hereinafter "sheet") is transported by a transport means 315 from any of sheet storage means 311, 312, 313, and 314 to the image formation unit 316. The toner image is transferred to the sheet and then fixed in a fixing unit 318, thereby forming the image on the sheet. The sheet with the image formed thereon is transported to a stapling tray 324 in the sheet postprocessing unit 304, stapled by a stapler 325, and discharged by a discharge means 320 into a discharge tray 323 attached to the exterior of the sheet postprocessing unit 304.
With this image-forming device, images may be formed on both the front and back of a sheet. When images are to be formed on both sides of a sheet, a sheet on the front side of which an image has been formed in the image formation unit 316 is turned over and stacked in an intermediate tray 319, and then transported back to the image formation unit 316, and after an image has been formed on the back of the sheet, it is transported to the sheet postprocessing unit 304.
As shown in FIG. 7, the image-forming device disclosed in Document II, like that in Document I, is composed of a sheet postprocessing unit 404 attached to the side of a main body 401 of the image-forming device.
In this image-forming device, an original placed on an original feed unit 403 is transported to and read by an optical unit 417, and a toner image is formed in an image formation unit 416 according to the Carlson process. A transfer sheet (hereinafter "sheet") is transported by a transport means 415 from a sheet storage means 411 to the image formation unit 416. The toner image is transferred to the sheet and then fixed in a fixing unit 418, thereby forming the image on the sheet. The sheet with the image formed thereon is transported to a stapling tray 424 in the sheet postprocessing unit 404, stapled by a stapler 425, and discharged through a turnover transport means 420 into a discharge tray 423.
In this image-forming device, when a stack of sheets which has undergone stapling, punching, or other postprocessing is discharged to the discharge tray 423, disarray of the stack is prevented by horizontal sliding and by allowing gravity to pull the stack down. Thus it is essential that the stack be discharged to the discharge tray 423.
In the image-forming device disclosed in Document III, as shown in FIG. 8, beneath an image formation unit 515 of the copy machine main body 501 is provided an intermediate tray 523 used for stacking sheets in double-sided copying. On the discharge side of the intermediate tray 523 is provided a filing device 535 having a stapler, which staples the stack of sheets stacked in the intermediate tray 523. In short, the intermediate tray 523 functions as the postprocessing tray of a sheet postprocessing device.
Accordingly, the image-forming device disclosed in Document III can be made more compact than conventional devices in which a sheet postprocessing unit was provided on the exterior of the copy machine main body.
However, in the device in Document I, as shown in FIG. 6, the sheet postprocessing unit 304 is provided on the side of the device, i.e., on the side of the main body, so that the stapling tray, discharge tray, etc. are provided on the side of the main body. Further, the hand-feed tray 314 is provided on the opposite side of the main body from the sheet postprocessing unit 304, so that the width of the hand-feed tray 314 and the width of the discharge tray 323 are added to the width of the main body. For this reason, the device in Document I has the disadvantage that the width of the device is increased, and the device as a whole is made larger.
In addition, during sheet postprocessing, the intermediate tray 319 (where the sheets with images formed thereon are stacked) and the stapling tray 324 (where the stack of sheets is stapled) are provided separately, thus wasting space and adding to costs.
In the case of the device in Document II, as shown in FIG. 7, since the discharge tray 423 is provided beneath the postprocessing tray 424, the postprocessing tray 424 is in the way, and the operator must bend down in order to check or remove the sheets discharged to the discharge tray 423. In other words, with the device in Document II, in order to view the sheets in the discharge tray 423, the user must look from the side of the device, and the discharged sheets are also difficult to remove. For these reasons, the device's operability is markedly impaired.
Again, with the device in Document III, as shown in FIG. 8, since the intermediate tray 523 used as a postprocessing tray is lower than the image formation unit 515, and the discharge tray 521 is provided even lower than the intermediate tray 523, the operator must bend down to remove the stack of sheets from the discharge tray 521, and such operation is troublesome, just as with the device in Document II.
Incidentally, among conventional image-forming devices such as copy machines and printers, some devices discharge a sheet of recording paper with an image formed thereon with the image side facing up, and other devices are provided with a turnover means (a means for turning over a sheet of recording paper) between the image formation unit and the discharge tray for discharging the sheet with the image side down. These devices can discharge the recording paper image side up by bypassing the turnover means, or discharge the recording paper image side down by transporting it through the turnover means.
However, the drawback of devices in which the turnover means is provided next to the image formation unit, and the discharge tray next to the turnover means, is that the width of the device as a whole is increased by the width of the turnover means.
In order to solve this problem, an image-forming device like that shown in FIG. 12 was proposed. In this image-forming device, a turnover means 682 is provided beneath the device main body 681, and a discharge tray 683 is provided on the side of the device main body 681. In this image-forming device, a sheet of recording paper with images formed thereon is discharged from a main body discharge hole 684 to a switching means 685. If the sheet is not to be turned over, the switching means 685 sends it to discharge rollers 686 and 687, which discharge it to the discharge tray 683.
If the sheet is to be turned over, on the other hand, the switching means 685 sends it to the turnover means 682 through a transport roller 688 and a pair of reversible rollers 689 and 690. While the rear end of the sheet in the turnover means 682 is held between the reversible rollers 689 and 690, the direction of rotation of the reversible rollers 689 and 690 is reversed. Thus the sheet is turned over by running it through a switchback. The sheet is then transported toward the discharge tray 683 by the discharge rollers 686 and 687, and discharged to the discharge tray 683.
However, in an image-forming device of this structure, each sheet must be transported into the turnover means until the rear end of the sheet clears the switching means, and then switched back and discharged to the discharge tray. Accordingly, when a number of sheets of recording paper are to be transported through the turnover means in succession, a sheet cannot be transported into the turnover means while the previous sheet is still in the turnover means, and the sheet transport interval must be equal to the time required for the switchback.
For this reason, when a number of sheets of recording paper are to have images formed on them in succession, the interval (time) between sheets required for switchback cannot be eliminated, and sheets with images formed thereon cannot be discharged at high speed, and thus efficiency of image formation cannot be improved.